The invention relates to a sprocket for a drive unit at the rear wheel of a bicycle. The sprocket may be one or more of a multi-sprocket assembly, wherein the multi-sprocket assembly may be installed on a freewheel body at the rear wheel hub of the bicycle and is drivable by a chain, such as a roller chain.
A freewheel body is formed by a driver of a cassette hub or by an outer ring of a freewheel that can be screwed to the hub sleeve. The ring is rotatably supported with respect to the hub axle and has a rotary coupling in the forward direction of rotation with respect to a hub sleeve and is freely rotatable in the backward direction of rotation with respect to the hub sleeve.
Sprockets at the rear wheel are main parts of the drive train of modern mountain bikes and road racing bicycles. Today, up to eleven sprockets are offered in a multi-sprocket assembly.
Sprockets belong to the group of highly stressed components and are parts that are subject to wear. Conventional sprockets comprise at their radially inner contour a drive profile at which a driving connection to a driver of a cassette hub or to the outer ring of a freewheel is established. The allocation of adjacent sprockets to each other in the circumferential direction, which is of vital importance during the process of shifting the chain from one sprocket to the adjacent sprocket, is determined by how the drive profiles of the sprockets are in engagement with the driver or the outer ring of the freewheel.
A new development trend in terms of multi-sprocket assemblies is weight reduction. In order to achieve this object while fulfilling the mentioned basic requirements, sprocket components are made from a lightweight material, e.g., from aluminum. The disadvantage here is the low hardness and the lower resilience against compressive forces on the component surface. This aspect has primarily an effect where the contact areas absorbing the compressive forces are made smaller with regard to their spatial expansion as this is the case for different types of shifting aids for supporting the shifting function along a shifting path. A shifting path extends along the roller chain changing from a sprocket to an adjacent sprocket, namely for shifting the chain from the smaller sprocket to the adjacent larger one (axially inwardly), one as well as from the larger sprocket to the adjacent smaller one (axially outwardly). Thereby, portions of the sprocket are omitted which are necessary for the engagement and the contact between a roller of the drive roller chain and the load-transmitting tooth flank. The features formed by omitting parts of the sprocket comprise shifting aids.
Thereby, a higher wear occurs on the thinner, weakened teeth which when using sprockets made of aluminum, for example, exceeds rapidly a threshold above which a reliable operation of the sprocket is no longer ensured.